This invention relates to an air-fuel ratio detecting apparatus for an internal combustion engine, and more particularly but not exclusively, it relates to an air-fuel ratio detecting apparatus for the engine of an automotive vehicle.
Recently, in order to accurately control the air-fuel ratio of the air-fuel mixture entering the cylinders of internal combustion engines, air-fuel ratio detectors have been installed on the exhaust manifolds of engines. Components of the exhaust gas of an engine which are correlated to the air-fuel ratio are detected by the air-fuel ratio detector, and the fuel supply is controlled by feedback so as to obtain a target value for the air-fuel ratio.
This type of air-fuel ratio detector generally has a sensing element and a heater which heats the sensing element to an activation temperature. A sensor of this type is described in Japanese Published Unexamined Pat. Application No. 60-58548.
The temperature of the exhaust gas of an internal combustion engine greatly varies depending on the operating state of the engine (indicated by parameters such as the engine rotational speed, the air intake rate, and the intake air pressure), temperature parameters such as the engine cooling water temperature and the intake air temperature, and the vehicle speed. Since an air-fuel ratio detector is disposed inside the exhaust manifold of an engine, it is exposed to the exhaust gas of greatly varying temperature. Therefore, in order to keep the temperature of the sensing element of the air-fuel ratio detector above an activation temperature and yet not overheat the sensing element, it is necessary to adjust the output of the heater for the sensing element in accordance with the exhaust gas temperature.
In a conventional air-fuel ratio detector, the output of the heater for the sensing element is controlled in accordance with the exhaust gas temperature as indicate by the air intake rate into the engine. When the air intake rate is below a prescribed rate, this is taken as an indication that the exhaust gas temperature is below a prescribed temperature and the heater for the sensing element is turned on. Conversely, when the air intake rate is above the prescribed rate, this is taken as an indication that the exhaust gas temperature is above the prescribed temperature and the heater for the sensing element is turned off.
However, this method of heater control is not sufficiently accurate, for as discussed above, the exhaust gas temperature of an engine depends on a large number of parameters besides the air intake rate, so that even at a constant air intake rate, the exhaust gas temperature can vary. Therefore, with this conventional method of heater control, the temperature of the sensing element can not be maintained constant.
Furthermore, over the entire operating range of the engine, variations in the exhaust gas temperature due to varying operating conditions are normally in excess of 800.degree. C. The range of variation of exhaust gas temperatures between when the heater of a conventional air-fuel ratio detector is on and when it is off is too large, the change in the temperature of the air-fuel ratio detector becomes too large, and the temperature dependence of the air-fuel ratio detector can no longer be ignored. Therefore, accurate detection of the air-fuel ratio of exhaust gas becomes extremely difficult.
A conventional air-fuel ratio detector also has the problem that the voltage of the battery of the vehicle is directly applied to the heater for the sensing element. During the operation of the vehicle, the battery voltage can vary, and so depending on the exhaust gas temperature, it may be impossible to maintain the temperature of the sensing element above its activation temperature.